1. Field of the Invention
The present invention relates to the structure of scavenging passages in a two-cycle engine. More particularly, the present invention relates to a two-cycle engine in which the cylinder section is narrowed.
2. Description of Background Art
The two-cycle engine is a known as an engine which is compact but can produce a high output because of its relatively simple structure and mechanism. The two-cycle engine has been widely applied to motorcycles, small boats, etc. In the known two-cycle engine, the valves are disposed near the exhaust ports, which are opened or closed in order to control exhaust gases. Therefore, the engine is allowed to operate under optimum conditions.
For example, the two-cycle engine disclosed in Japanese Patent Laid-Open Publication No. Hei 7-71279 or No. Hei 7-180556 includes exhaust control valves disposed on the inner walls of the exhaust passages near the upper edges of the exhaust ports for opening or closing the exhaust ports. When the engine is operating at a low speed or under a low load, the exhaust control valves substantially fully close the exhaust ports. This causes the pressure in the cylinders to be optimally cotrolled, fresh air in the combustion chambers to be activated using thermal energy of already burnt gasses remaining therein, combustion in an active and hot atmosphere in which self-ignition combustion is carried out to be realized, and fuel economy to be improved and purification of exhaust gases to be promoted.
Furthermore, Japanese Patent Laid-Open Publication No. Sho 56-56915 exemplifies a two-cycle engine in which the exhaust control valves for varying exhaust emission timing are positioned on the inner walls of the exhaust passages near upper edges of the exhaust ports. When the engine is operating at a high speed, the exhaust control valves are fully opened to advance the timing for opening the exhaust ports (i.e., exhaust emission timing), thereby promoting discharge of exhaust gases. Conversely, when the engine is operating at a low speed, the exhaust control valves are nearly closed to retard the exhaust emission timing and prevent fresh air from blowing through the cylinders.
In the known two-cycle engine, the scavenging passages are usually formed on a side of the cylinder block. When a plurality of cylinders are juxtaposed, spaces between the cylinders (i.e., the width of a cylinder section) inconveniently become too large. Furthermore, when the exhaust control valves are provided, it is required to simplify the mechanism for activating the exhaust control valves, to improve assembling efficiency, and facilitate maintenance of the engine.
In order to meet the foregoing requirements, two-cycle engines including exhaust control valves for varying the exhaust emission timing have been proposed in Japanese Patent Laid-Open Publications No. Hei 3-74519, No. Hei 3-74520, and No. Hei 7-97912.
In one of such proposals shown in FIG. 7, exhaust passages 101 are inclined in the same direction with respect to a plane which is orthogonal to the center axis 100 of the cylinders (i.e., the axis along which the cylinders are juxtaposed). Furthermore, a pivot 103 rotated by a servo-motor 102 is arranged in parallel with the cylinder center axis 100. Exhaust control valves 105 positioned at upper edges of the exhaust ports 104 are supported in common by the pivot 103, thereby simplifying the mechanism for activating the exhaust control valves. In addition, scavenging passages 107 are formed in the cylinder block 106 in order to supply fresh air (an air-fuel mixture) to the combustion chambers from the crankcase. The scavenging passages are inclined in the same direction with respect to the plane which is orthogonal to the cylinder center axis 100. Therefore, the exhaust passages which are sandwiched between the exhaust ports are arranged in a staggered manner in order to reduce the width of the cylinder section.
In the example shown in FIG. 7, the pivot 103 is inclined with respect to an opening direction of the exhaust ports 104, so that the rotating exhaust control valves 105 cannot reliably close the exhaust ports 104. This prevents the exhaust emission timing from being precisely controlled. In the example, the shape of the exhaust control valves 105 are constructed to overcome the foregoing problem. However, this construction is not advantageous since the exhaust control valves become too large and the spaces for housing them become complicated. This results in an increase in manufacturing cost, and reduced efficiency in assembling and maintaining the engine.
Since the pivot is inclined with respect to the opening direction of the exhaust ports 104, the pivot 103 inevitably is very large. Furthermore, the exhaust control valves are longer between their bases and tips. Such lengthened members have reduced rigidity. Therefore, it is difficult to precisely control them.
The foregoing holds true for exhaust control valves which enable combustion under an active and hot atmosphere.